1. Field of the Invention
The present invention relates to a semiconductor device including a MOS transistor and a protection device integrated therein and a method of manufacturing the same. In particular, the present invention relates to a semiconductor device including a protection device with good constant-voltage characteristics and being capable of simplifying a process, and to a method of manufacturing the same.
2. Description of the Related Art
In a discrete device of a MOS-type semiconductor, a gate oxide film is the most important part, and is the most fragile part at the same time. As measures for protection of the gate oxide film, a Zener protection diode is provided to protect the gate oxide film when an excessive current, an excessive voltage, static electricity, or the like is externally applied to a gate terminal.
FIGS. 14A and 14B illustrate a conventional semiconductor device. FIG. 14A is a planar view, and FIG. 14B is a cross-sectional view taken along the line e-e in FIG. 14A.
The semiconductor device is formed by integrating, for example, a metal oxide semiconductor field-effect transistor (MOSFET) 56 and a protection diode 57 on a single chip. In an element region 55, the MOSFET 56 constituted by multiple cells is arranged. A gate electrode of the MOSFET 56 is led out of the element region 55 through a polysilicon layer, and is connected to a gate pad electrode 58.
A semiconductor base is formed by stacking an n− type semiconductor layer 42 on an n+ type silicon semiconductor substrate 41 and then forming a channel layer 43 thereon. Inside of each trench 44 which penetrates the channel layer 43, a gate oxide film 45 is formed, and a gate electrode 46 is embedded. On a surface of the channel layer 43, a body region 49 and a source region 48 are formed. On the gate electrode 46, a source electrode 47 is formed with an interlayer insulating film 50 interposed therebetween.
The protection diode 57, which is arranged below the gate pad electrode 58, is a bidirectional Zener diode formed by connecting multiple pn junction diodes between p type regions 51 and n type regions 52. One end of the protection diode 57 is connected to the source electrode 47 of the MOSFET 56, while the other end is connected to the gate pad electrode 58. A resistor 53, which is made of polysilicon, has one end connected to the gate pad electrode 58 and the other end connected to a polysilicon layer 54 which is connected to the gate electrode 46 of the MOSFET 56.
The breakdown voltage of the protection diode 57 is set slightly lower than the breakdown voltage of the gate oxide film 45 of the MOSFET 56 (breakdown voltage of the oxide film 45). With this configuration, in the case where an excessive load is applied to a gate terminal, an excessive current is bypassed to the protection diode 57 in order to prevent an excessive load from being applied to the gate electrode 46, and the gate oxide film 45 is thereby protected.
This technology is described for instance in Japanese Patent Application Publication No. 2002-43574.
The conventional protection diode 57, as shown in FIG. 14B, is a polycrystalline pn junction Zener diode formed by doping polycrystalline silicon with p type and n type impurities through solid-phase diffusion, ion implantation, or the like. However, a Zener diode having a polycrystalline pn junction does not achieve good characteristics as a constant voltage diode compared to that having a single-crystalline pn junction.
FIG. 15 shows I-V characteristics of a polycrystalline pn junction.
As shown in FIG. 15, the polycrystalline pn junction does not have steep I-V characteristics (breakdown voltage characteristics), but has gradual characteristics. Accordingly, in the case where an excessive load is externally applied thereon, an excessive current cannot fully be bypassed by use of the protection diode 57 under heavy load although being capable of dealing with a small load. Accordingly, a voltage of the breakdown voltage of the gate oxide film 45 or above is applied to the gate electrode 46, which results in destruction of the gate oxide film 45.
Meanwhile, the gradual I-V characteristics indicate that a leakage current is excessive before the breakdown voltage is reached. Accordingly, connection of the protection diode 57 formed by a polycrystalline pn junction lowers the switching characteristics of the MOSFET 56.
In other words, the film thickness of the gate oxide film 45 is limited by the performance of the protection diode 57. Therefore, the gate oxide film 45 has to be made thicker than necessary in consideration of an excessive voltage to be applied. Such an over-margin is one of the causes of lowering device performance of the MOSFET 56.
Furthermore, the above-described protection diode 57 is formed by patterning polycrystalline silicon on a surface of a substrate, and then doping predetermined regions respectively with p type impurity and n type impurity. To be more specific, a step of forming a protection diode is added in a manufacturing process of the MOSFET 56. Accordingly, the process is more complicated and the cost thereof is increased.